1. Field of the Invention
The present invention relates generally to vacuum deposition, and more particularly to a thin film sputtering machine having a batch process path and a serial sputtering chamber.
Sputtering systems generally allow the deposition of a film of a selected material onto a substrate. Sputtering has found increasing applications in the fabrication of integrated circuits from silicon wafers, and particularly in the production of magnetic recording media.
Modern sputtering systems generally include a magnetron which produces a magnetic field in front of the target. The magnetic field intensifies and confines the plasma to a stable region in front of the target, greatly improving the efficiency and deposition rates of the process, and also reducing the heating of the substrate.
There are two types of systems generally used to sputter magnetic recording disks. The first, referred to as a "disk-by-disk" system, produces sputtered films of the highest possible quality. Disk-by-disk systems utilize a serial process method in which each substrate is placed individually between circular magnetrons where a single layer is deposited. The substrate moves sequentially to subsequent sets of circular magnetrons until all desired layers have been deposited. Each substrate must be individually aligned with the sputtering target at a fixed distance and sputtered at a fixed position. The main advantage of the disk-by-disk systems is the circumferential uniformity of the layer structure, which results in uniform magnetic recording characteristics.
Disk-by-disk sputtering systems suffer from serious disadvantages. The serial sputtering process greatly increases costs, in part from the need for a complex handling system to move the disks one at a time from one set of sputtering sources to another. These handling systems also greatly reduce the sputtering system reliability. No increase in through-put is achieved with smaller disks, as a sputtering machine only produces a single product at any given time. Furthermore, the sputtering chamber must be evacuated and back-filled each time it is opened to introduce another substrate. In addition, the mechanical movement from one chamber to another, sealing the chambers, and re-establishing the correct gas environment take a considerable amount of time and substantially limit the throughput rate of known disk-by disk systems.
Other factors limiting the effectiveness of serial sputtering are the requirements of providing separate sputtering chambers for each deposition material, and the need to isolate the deposition materials and chambers from each other during the sputtering process. Typically, the gases used in sputtering some of the layers are different from those used for other layers, and cross-contamination of these gases could reduce the quality of the deposited film structures. For a typical thin film disk three layer sputtering process, the first underlayer and the magnetic alloy recording layer are preferably sputtered using argon gas, while the carbon overlayer is sputtered using a hydrocarbon/argon or nitrogen/argon gas mixture. This hydrocarbon or nitrogen gas can significantly reduce the coercivity of the magnetic layer if present in the underlayer and magnetic layer process chambers, and/or if absorbed onto the underlayer before the magnetic layer is deposited. Thus, known sputtering chambers are sealed during the sputtering process. However, environment and then (after deposition) turning off the gas, evacuating the chamber, and unsealing the chamber, causes significant delays in the operation of the system. Furthermore, magnetic recording characteristics benefit from a magnetic recording layer sputtered immediately after sputtering an underlayer on the recording media substrate. As noted above, however, the sputtering target and the substrate are preferably axially aligned within the sputtering chamber to provide a uniform film thickness. Thus, the time spent moving the disk from one chamber to the next, aligning the disk, and then sealing the chamber and re-establishing the gas environment, also allows time for the first film layer to be exposed to any contaminating gas, and thereby reduces the quality of the film.
The second type of sputtering system which is generally used to sputter magnetic recording disks is referred to as a "pass-by" system. Pass-by systems utilize a steady-state process in which several substrates are loaded on a pallet and the pallet passes between a series of long rectangular magnetrons which sequentially deposit the desired layers on both sides of the disks. These magnetrons must be separated to prevent cross-contamination. The main advantage of pass-by systems is that several disks an be placed on a pallet and sputtered at one time, and as the size of the disk decreases, more disks can be sputtered at the same time.
Steady-state sputtering deposition processes, in which substrate disks pass through a scanning sputtering stream, result in an undesirable anisotropy of the deposited film. First, pass-by systems deposit a non-uniform film layer due to inherent thickness variations when passing a circular disk by a rectangular target. Additionally, pass-by systems exhibit an angle of deposition variation as the disk approaches the sputtering zone, is directly in front, then leaves the sputtering region. Specifically, some portion of the disk will be sputtered while the sputtering stream is scanning the substrate radially towards or away from the center of the disk. Alternative portions of the disk are sputtered while the sputtering stream is scanning tangentially across that portion of the disk. These non-uniformities result in modulation due to variation in magnetic properties circumferentially around the disk.
Although the anisotropy of pass-by systems can be alleviated to some extent by rotating the disk during the sputtering process, this requires the presence of mechanical moving parts in the harsh environment of the sputtering chamber, and often prevents simultaneous sputtering of both sides of a substrate disk. Disk handling machinery costs are another disadvantage of pass-by systems; to take advantage of the increased throughput when reducing the substrate size, the handling systems used to load and unload the substrates from the pallets often must change to faster, more accurate automated machines which are very expensive. These additional system components decrease the cost advantages of pass-by systems.
A disadvantage of both the pass-by and disk-by-disk sputtering systems is an inherent delay between sputtering successive layers while the disks are moved from one location to another. Magnetic disks generally involve two or more layers, and the quality of the films of upper layers depends on the quality of the lower layers. In particular, an underlayer of chromium or chromium alloy is often used with a cobalt alloy magnetic layer over it. A high coercivity of the cobalt layer is important to achieve high density recording properties. However, if the chrome layer gets an oxide layer on it before the cobalt alloy layer is deposited, the coercivity decreases. Any delay between the end of the chrome sputtering and the beginning of the cobalt sputtering allows some oxide to form, even in a high vacuum, because there is always some residual oxygen in any vacuum system. Thus, the delay inherent between deposition of subsequent layers in both pass-by and disk-by-disk systems limits the recording properties of magnetic recording media.
Another disadvantage of known pass-by and disk-by disk systems is that they are often limited in their throughput rate by the heating and cooling processes. For example, very high heating rates can cause warpage of the substrate. On the otherhand, very high cooling rates are primarily achieved by having a cold surface very close to the disk surface, which is difficult to do without occasionally touching the surface of the deposited film, thereby destroying the disk. Clearly, suing reduced heating and cooling rates extends the process time and thereby reduces throughput rate. It would be desirable to provide some or all of the heating and cooling in a batch process so that the rate per disk could be lowered without reducing the throughput rate for the entire system.
For these reasons it would be desirable to provide improved sputtering deposition methods and devices which provide the benefits of batch sputtering processes, but which would not compromise the sputtering film quality. It would be particularly desirable to provide a flexible sputtering machine which could produce a variety of sputtered layer structures. It would be especially desirable if such a sputtering machine could produce a sputtered layer of a first material, and could immediately sputter a second layer of a second material over the first layer. It would be best if such capabilities could be realized without a large, complex, expensive sputtering apparatus having a large number of moving mechanical parts within the harsh environment of the sputtering chamber.
2. Description of the Background Art
U.S. Pat. No. 5,316,864 describes a steady-state production sputtering machine and describes the anisotropy of "pass-by" recording media. U.S. Pat. No. 4,869,802 describes an apparatus for rotating the substrate during the sputtering process. U.S. Pat. No. 5,228,968 describes a cathode sputtering system having an axial gas distribution. U.S. Pat. No. 5,180,478 describes a sputtering source having a coolant path behind a target. U.S. Pat. No. 4,786,564 describes magnetic recording media which include a sputtered underlayer over which the magnetic layer is immediately sputtered.